As has been known for a long time, modern automatic transmissions in motor vehicles are controlled by an electronic transmission control (EGS) which for automatic selection of a stored shifting program which describes, for example, a known detection of a driver type, a detection of the environment, a detection of a driving situation, or a detection of a manual engagement and for gear selection according to situation communicates with other calculating and control units of different aggregates. The operating points at which the electronic transmission control issues a command for upshifting or downshifting to the hydraulic system, is determined by shifting characteristic lines stored in the electronic transmission control and usually shown as a function of a vehicle velocity and of a position of the accelerator pedal. In older motor vehicles, the position of the accelerator pedal is mechanically detected via a throttle valve position while in modern motor vehicles the accelerator pedal position is detected with “electronic gas”.
Life experience teaches that in certain driving situations the guide rod of a motor vehicle quickly removes the foot from the accelerator pedal. In case of a sufficiently high velocity of the vehicle, an upshift to a more economical gear such as from third to fourth gear often takes place. In the coasting operation, the rotational speed of the engine at the same time drops with the upshift.
Such an upshift is obstructive in a required or desired acceleration that follows, since at least one time-consuming downshift has to be carried out in order to make a desired engine input available.
In some driving situations, however, it is not only a reduction in driving comfort that such upshift and subsequent downshift represent, for if this occurs during a brief pedal release or gas release also called “FastOff”, it can lead to a danger situation, for example, in case of a discontinued or briefly interrupted overtaking operation since, in the first place, no desired deceleration of the vehicle appears during the “FastOff” and, secondly, the desired rotational speed range with maximum input does not appear, for example, when the overtaking operation is continued.
EP 0 574 965 A1 has disclosed a method for detecting a shift signal from a shift characteristic field with which an upshift of the transmission in the coasting operation has to be prevented and, should that be the case, under high transverse acceleration. To this end is maintained the overrunning of upshift lines of the shift field by the operating point resulting from the momentary vehicle velocity and throttle valve position. The position of the accelerator pedal is measured and, when exceeding the upshift characteristic line, it is converted to an accelerator pedal velocity. The measured value of the accelerator pedal velocity thus obtained is compared with a limiting value stored in the transmission control, an upshift shifting signal is issued when falling below said limiting value. The accelerator pedal velocity is thus a criterion of whether the transmission upshifts. It is further proposed in this method that the upshift operation be suppressed when a transverse acceleration limiting value has been exceeded.
However, this method in which an upshift block is released depending on a negative gradient of the accelerator pedal angle has proved disadvantageous in the practice in the sense that the upshift block is not sensible enough in the case of a brief release of the pedal. It has been found that the upshift block is often introduced too early or too late, or that the driving has been carried out for too long or too short a time at a rotational speed too high for the actual operating situation. Both involve a clear reduction in the driving comfort.
One other method for transmission control has become known from DE 41 20 566 where for activation of the upshift block, in addition to the pedal angle gradient, the criterion “coasting operation” must be detected via a push-pull characteristic line. The mode of the upshift block is left when a “traction operation” is detected and/or a deceleration time has elapsed. But this has proved insufficient for ensuring a correct shifting behavior in the event of spontaneous gas release.
In DE 198 49 059 A1 has been described an improved method in which a spontaneous gas/pedal release (FastOff) is detected and an upshift block is activated when a pedal position gradient is smaller than a pedal position gradient threshold stored in a characteristic field as a function of a pedal position value and of a driver-type evaluation meter. The mode of the upshift block is active until a traction operation is detected when the actual pedal position value exceeds a pull-push characteristic line stored in a characteristic field as a function of a value equivalent to an engine rotational speed and of an acceleration potential value equivalent to a road inclination value.
Complementarily or alternatively, the mode of the upshift block is active until a time has elapsed after a defined time period, the mode of the upshift block being exited after a first period when the pedal position gradient in the first period remains below a positive pedal position gradient threshold and a second period is started when the positive pedal position gradient threshold is overrun and again underrun. The mode of the upshift block after lapse of the second period being exited when the positive pedal position gradient threshold has not again been overrun in the second period.
In this known method, the criteria for the entry and the exit of the “FastOff” function have thus been broadened and made precise. But the standard of fixed time periods always has attached the disadvantage that here the actual vehicle behavior cannot be sufficiently taken into account so that the mode of the upshift block is kept for too short or for an unduly long time, as the case may be.
This invention is based on the problem of making available for control of an automatic transmission of a motor vehicle in the event of a spontaneous gas/pedal release, a method where an upshift block which, after a spontaneous pedal release and a deceleration of the vehicle velocity, allows, as long as it is activated, a great spontaneity for a subsequent acceleration depending on how this fits the driving situation.